Reform movements of the early 20th century promoted, among other things, greater influence of the people in government. The effects of the Great Depression were not as devastating in California as in most other parts of the country but were felt nonetheless. Migrant farmworkers from the Dust Bowl of the Great Plains flocked to the state to seek work, a situation that caused widespread social unrest. Depression conditions gave rise to a number of social welfare schemes, including the End Poverty in California (EPIC) reform movement, organized by muckraking author and gubernatorial candidate Upton Sinclair. The Democratic Party gained strength during the Depression era. Nevertheless, Republicans dominated the statehouse during the first half of the 20th century (the Democrats had control only during 1939–43). Notable among the Republican governors was Earl Warren, who resigned in 1953 to become chief justice of the United States and who was the first person of his state to hold the office.

In 1958 a Democratic victory installed Edmund Brown as governor. The Republican defeat reflected a national trend, and Democrats not only won gubernatorial and U.S. senatorial races but also received a majority of seats in both houses of the state legislature for the first time in the 20th century. In 1966 Republican Ronald Reagan, a former actor who in 1981 would become president of the United States, replaced Brown. From the 1960s on, the state seesawed between Democratic and Republican leadership.

Meanwhile, the economy advanced and the population grew. An influx of immigrants first moved to southern California about 1900, spurred by citrus, oil, and some wariness of San Francisco in the north after the earthquake and fire of 1906 (see San Francisco earthquake of 1906). Land booms came and went. Agriculture in inland valleys and industry in the cities increased. During World War II aircraft plants and shipyards expanded, and in the 1950s and ’60s research and educational facilities burgeoned as the movement of people to the West Coast came to include an unusual share of scientists and academicians who had migrated to the United States during World War II.

The 1970s and ’80s were marked by shifting demographics. New urban centers emerged, and smaller cities experienced the most rapid rate of growth. The population of San Diego’s metropolitan area rose past two million and included the highest proportion of college graduates of any of the 10 most populous North American cities. There was also rapid growth in new suburban areas near San Jose, Sacramento, and Riverside. For a century San Francisco had served as a financial and corporate center, but most major corporations had relocated their headquarters to more populous southern California by the late 1980s.

California’s population continued to increase rapidly throughout the 1980s, after which a national economic recession caught up with the population boom. In the early 1990s the U.S. government decreased defense spending, shrinking the state’s expansive aerospace and military contracting industry. At the same time, California was plagued with a series of natural disasters—floods, fires, droughts, and earthquakes—which contributed to diminishing the allure of the Golden State. In 1994 California began moving toward economic recovery in the footsteps of the rest of the country.

This recovery was spurred by two related phenomena: the explosive growth of the high-technology sector, concentrated at first in Silicon Valley but later widely distributed throughout the state; and the arrival of hundreds of thousands of highly educated workers from other states and countries to work in the industry. The new arrivals contributed to a more cosmopolitan and diverse population. This new prosperity benefited many Californians, but it also helped underscore the differences between the affluent and the working class, many of whom were immigrants.

California’s ethnic and racial diversity has sometimes resulted in uneasy relations between the groups. In the 19th century the Chinese first became a target of discrimination, which persisted into the 20th century, and as other ethnic groups made California their home, tensions sometimes flared. During World War II, nearly 100,000 Japanese—including many U.S. citizens—were interned in relocation camps out of fear that their allegiance would lie with Japan and not the United States (in 1988 the federal government officially apologized for these actions and authorized payment of $20,000 to every individual who had been forced to move). California’s African American population increased dramatically after World War II, and in 1965 and 1992 riots erupted, the former owing mainly to economic grievances and the latter to what was perceived by many African Americans as discrimination in the criminal justice system after several white officers were acquitted in the beating of an African American motorist. In addition, the position of the large number of Hispanics who migrated to California was often tenuous; many worked under hard conditions for little pay, and others had immigrated to the United States illegally. The issue of illegal immigration was of particular importance in California; in 1994 Californians approved a ballot initiative (Proposition 187) to deny illegal immigrants social services (later overturned by a federal court). The increasing political strength of Hispanics in the state (Antonio Villaraigosa became Los Angeles’s first Hispanic mayor since the 19th century) sometimes put them at odds with African Americans.

Politically, California has often served as a laboratory for direct democracy, and in the early 21st century the state ranked first nationally in the number of initiatives that appeared on its ballots. In fact, since the 1960s the number of ballot initiatives has increased fivefold on matters including health care, the environment, taxes, and other special issues. In May 2008, in a landmark decision, the California Supreme Court struck down a state law, based on a ballot initiative, that had banned same-sex marriage.

That decision was overturned in November 2008, when California’s voters approved Proposition 8, a statewide ballot initiative that banned same-sex marriage. As a result of the proposition, a new amendment was added to the state constitution specifying that only marriage between a man and a woman would be recognized by the state. Several lawsuits challenging the proposition were filed. Although the ban was upheld by the California Supreme Court in May 2009, it was ultimately overturned in 2013, following a Supreme Court ruling that effectively let stand a federal district court’s ruling that Proposition 8 was unconstitutional.

Meanwhile, California’s economy was shaken again by a nationwide—and worldwide—economic downturn that reached crisis in 2008–09. Faced with a multibillion-dollar budget deficit, Gov. Arnold Schwarzenegger declared a fiscal emergency in December 2008. In a special statewide election held in May 2009, voters rejected five of six ballot measures that were meant to help close the staggering deficit.